The present invention relates to a system for providing pressurized fluid to a plurality of load circuits, and more particularly, to such a system in which one of the load circuits is given priority, with all flow not being used by the priority circuit going to the other (auxiliary) load circuit.
The present invention is equally adapted to any arrangement in which pressurized fluid is communicated to a priority load circuit and an auxiliary load circuit by a load sensing priority flow control valve, in response to a load pressure signal indicating the demand for fluid by the priority load circuit. However, the invention is especially advantageous in arrangements in which the priority load circuit comprises a vehicle hydrostatic power steering system and the invention will be described in connection therewith.
Load sensing priority flow control systems are used in many applications in order to provide pressurized flow to multiple load circuits from a single source (pump), partly to make the overall system less expensive, and partly to minimize the energy consumption (i.e., the load on the vehicle engine). Typically, in such vehicle applications, the vehicle engine is set to idle at a particular, minimum speed (idle speed), such as 600 rpm. Based upon this predetermined idle speed of the engine, the hydraulic pump and various other components are selected and sized such that the fluid output of the pump, at engine idle, is sufficient to satisfy at least a minimal demand for fluid by the priority load circuit. For example, if the priority load circuit is the vehicle power steering system, the various hydraulic components must be sized such that, at engine idle, the pump provides at least sufficient fluid to steer the vehicle at a rate which is consistent with safety requirements and through any range of loads likely to be experienced. Because one of the conditions frequently encountered is steering against the travel limits, it is preferable to limit the steering system to a pressure substantially below the maximum system pressure available to the auxiliary load circuit.
In some systems of the type described above, when the engine is operating at idle speed, the engine doesn't have enough power to drive the fluid pump to provide adequate pressure, even at a low flow rate, to operate the auxiliary load circuit, without stalling the vehicle engine. If the vehicle operator attempts to actuate the auxiliary load circuit with the engine at idle speed, there will normally not be sufficient fluid output from the pump, in terms of flow or pressure, to actuate the auxiliary implements in the manner desired. However, the actuation the auxilary load circuit can impose a load great enough on the vehicle engine to stall the engine, which, if permitted to occur periodically, would result in very inefficient operation of the vehicle, and in some cases, may result in a potentially dangerous situation with regard to the control of heavy moving loads.
Various anti-stall control arrangements have been developed for vehicle hydraulic systems, especially for use in connection with hydrostatic transmission systems which include variable displacement pumps. Typically, the engine speed is either sensed directly, such as by means of a magnetic pickup which reads shaft rpm, or is sensed indirectly by means of a pressure drop across a fixed orifice. In either case, known anti-stall controls normally involve either reducing the displacement of the pump to reduce the load on the engine, or increasing the engine throttle setting. In either case, known anti-stall control systems are normally fairly complex and expensive, and their use generally results in various operating problems such as higher power loss at optimum engine speeds.